Circuit arrangement comprising two successive valve amplifying stages



June 4, 1963 w. ASCHERMANN 3,092,785

CIRCUIT ARRANGEMENT COMPRISING TWO SUCCESSIVE VALVE AMPLIFYING STAGES Filed Jan. 21, 1960 INVENTOR W. ASCHERMANN AGEN ,9 ,73 Patented June 4, 1963 3,092,785 CRCUET ARRANGEMENT COMPREING TWG SUCCESSTVE VALVE APHLIFYING STAGES Wilfried Aschermann, Hamburg-Harburg, Germany, as-

signor to North American Phiiips Company Inc, New

York, N.Y., a corporation of Beiaware Filed Jan. 21, 1966, Ser. No. 3,829 Claims priority, application Germany Mar. 17, 1959 3 Claims. ((31. 330-178) In a circuit arrangement comprising two successive valve amplifying stages a certain degree of coupling always occurs between the output electrodes, particularly the anodes of the valves across the stray capacity C,,, which is particularly high, if the valve systems are housed in the same bulb. This involves an undesirable transfer, which is particularly troublesome, when a control-device is connected between the two stages, which device is also intended to provide a complete blocking of the transmission. This applies, for example, to low-frequency amplifiers comprising a sound volume potentiometer. In the said position for blocking of the transmission oscillations particularly of the higher frequencies will nevertheless reach the output.

Experimentation leading to the present invention has shown that neutralization can be obtained in a simple manner, whilst, in addition, the capacity C between the output electrode of the first valve and the input electrode of the second valve and the capacity C between the output electrode and the input electrode of the further valve are to be taken into consideration.

Briefly stated, in accordance with the present invention, a pair of amplifying devices are cascade connected by means of a potentiometer. Stray capacitances exist between the electrodes of the two devices. In order to neutralize the effect of these capacitances, so that signal transmission will not occur between the output of one device and the output of the other device by way of these capacitances, a resistor R is inserted in series with the input electrode of the second device in the cascade connection. 'Ihe resistor R has a value substantially equal to:

a SC where C,, is the stray capacitance between the output electrodes of the two devices, C is the stray capacitance between the output electrode of the first device and the input electrode of the second device, and S is the mutual conductance of the second device. With this arrange ment, signal currents transmitted by way of stray capacitance between the output electrodes of the two devices will be equal and opposite to signal currents at the output electrode of the second device resulting from amplification of signal currents transmitted from the output electrode of the first device to the input electrode of the second device by way of stray capacitances.

The invention will be described more fully by way of example with reference to the drawing, in which:

FIG. 1 is a circuit diagram of a two-stage amplifier embodying the neutralizing arrangement of the present invention; and

FIG. 2 is an equivalent circuit of the circuit of FIG. 1 with the interelectrode capacitances trans-formed to a star connection.

The anode circuit of a first valve amplifying system 1, which is enclosed in the same bulb with a second amplifying valve system 2, includes a working resistor 3, connected to the positive terminal of the supply source. The oscillations occurring across this resistor are fed via a blocking capacitor 4 to a potentiometer P, which is otherwise connected to earth. The tapping thereof is connected via a second blocking capacitor 5 to the grid of the valve 2 and via a leakage resistor 6 to earth. The cathode circuit of the valve 2 includes in known manner a cathode combination of a resistor 7 and a parallel capacitor 8. In the anode circuit of the valve is included a working resistor 9, from the terminal 10 of which are obtained the output oscillations.

The arrangement so far described is of known type.

Particularly with multiple valves 2. coupling across the stray capacity C occurs between the output electrodes, in this case the anodes, of the valves. This capacity becomes particularly manifest when the tapping of the potentiometer P is turned down and connected to earth, so that no oscillations should occur at the output. The high frequencies are, however, transmitted to a very appreciable extent particularly via the capacity C,,.

Between the output electrode of the valve 1 and the input electrode of the valve 2, as well as between the output and input electrodes of the valve 2 prevail further stray capacities C and C respectively, which, together with the capacity C constitute a delta connection. This delta connection may be varied in known manner into a start connection, in which case a compensation of the oscillations fed to the output resistor 9 via the said capacities is obtained, if in accordance with the invention between the input electrode (grid) of the valve 2 and the output circuit of the first valve 1 is connected a resistor R.

After the said change into the star connection, an equivalent circuit diagram as shown in FIG. 2 with the voltage and current values indicated herein is obtained; the output voltage U disappears, when the currents I and I are equal and opposite each other. On the condition that the capacitative impedance of the capacitor C should be materially higher than the value of the ohmic resistor R the following relations may be established:

Herein designates a the amplification factor and r the internal resistance of the valve 2.

if it is considered that the sum of the two currents I and I should be zero, it applies that is obtained.

By transforming the capacities back to a delta connection,

When finally the aforesaid conversion of the delta connection of the stray capacities into the star connection of the capacities C C and C has been carried out and when the initial values are reintroduced, the aforesaid condition is finally obtained.

Thus the compensation can be, in fact, realized and no voltage will arrive at the output terminal 10 via the stray capacities. This condition applies in the first place to higher frequencies. With lower frequencies also the influence of the coupling capacitor 5 becomes manifest.

However, as long as the impedance of this capacitor does not exceed approximately one fifth of the neutralisation resistor R, no interference is experienced even with lower frequencies, moreover, because in this range also the transmission via the comparatively small stray capacities is only very slight.

What is claimed is:

1. In a circuit of the type having first and second cascade connected amplifying devices each having an input electrode and an output electrode, and in which mutual interelectrode capacitances exist between the electrodes of said devices, means for coupling the output electrode of said first device to the input electrode of said second device, said coupling means comprising potentiometer circuit means connected to the output electrode of said first device, resistor means, means connecting said resistor means between the tap on said potentiometer and the input electrode of said second device, said resistor means having a value R determined by the expression:

s so.

where C is the interelectrode capacitance between said output electrodes, C is the interlectrode capacitance between .the output electrode of said first device and the input electrode of said second device, and S is the mutual conductance of said second device, input circuit means connected to the input electrode of said first device, and output circuit means connected to the output of said second device.

2. A cascade connected amplifying circuit comprising a first amplifying device having an output electrode, a second amplifying device having an output electrode, an input electrode, and a common electrode, potentiometer circuit means having first and second fixed terminals and a variable tap, means connecting said first terminal to the output electrode of said first device, means connecting said second terminal to a reference potential, first and second resistor means serially connected in that order between said input electrode and said reference potential, means connecting said common electrode to said reference potential, capacitor means connected between said tap and the junction of said first and second resistors, said 4 first resistor having a value R determined by the expression:

n SCK1 where C is the interlectrode capacitance between said output electrodes, C is the interelectrode capacitance between the output electrode of said first device and the input electrode of said second device, and S is the mutual conductance of said second device, input circuit means connected to the input electrode of said first device, and output circuit means connected to the output of said second device.

3. An amplifier circuit comprising a first amplifying device having a first input electrode, a first output electrode, and a first common electrode, a second amplifying device having a second input electrode, a second output electrode, and a second common electrode, a first resistor having a variable tap, means connecting the fixed terminals of said first resistor between said first output electrode and first common electrode, and means connecting the tap of said first resistor between said second input and second common electrodes comprising a second resistor, means connecting said second resistor between said tap and said second input electrode, said second resistor having a value R determined by the expression:

where C,, is the interelectrode capacitance between said output electrodes, C is the interelectrode capacitance between the output electrode of said first device and the input electrode of said second device, and S is the mutual conductance of said second device, input circuit means connected to said first input electrode, and output circuit means connected to said second output electrode.

References Cited in the file of this patent UNITED STATES PATENTS 2,121,150 Jarvis June 21,1938 2,229,812 Maitland Ian. 28, 1941 2,298,987 Thomson Oct. 13, 1942 2,519,359 Dean Aug. 22, 1950 2,660,614- Oliver Nov. 24, 1953. 2,783,466 tDunn Feb. 26, 1957 

1. IN A CIRCUIT OF THE TYPE HAVING FIRST AND SECOND CASCADE CONNECTED AMPLIFYING DEVICES EACH HAVING AN INPUT ELECTRODE AND AN OUTPUT ELECTRODE, AND IN WHICH MUTUAL INTERLECTRODE CAPACITANCES EXIST BETWEEN THE ELECTRODES OF SAID DEVICES, MEANS FOR COUPLING THE OUTPUT ELECTRODE OF SAID FIRST DEVICE TO THE INPUT ELECTRODE OF SAID SECOND DEVICE, SAID COUPLING MEANS COMPRISING POTENTIOMETER CIRCUIT MEANS CONNECTED TO THE OUTPUT ELECTRODE OF SAID FIRST DEVICE, RESISTOR MEANS, MEANS CONNECTING SAID RESISTOR MEANS BETWEEN THE TAP ON SAID POTENTIOMETER AND THE INPUT ELECTRODE OF SAID SECOND DEVICE, SAID RESISTOR MEANS HAVING A VALUE R DETERMINED BY THE EXPRESSION: 